1. Technical Field
The present invention relates to a method of etching semiconductor nanocrystals, and more particularly to a method of etching semiconductor nanocrystals which includes dissolving semiconductor nanocrystals in a halogenated solvent containing phosphine to thus induce anisotropic etching of the surface of semiconductor nanocrystals or dissolving semiconductor nanocrystals in a halogenated solvent containing either a primary amine or a primary amine and phosphine and then photoexciting them to thus induce isotropic etching of the surface of the nanocrystals, thereby reproducibly controlling properties of semiconductor nanocrystals including absorption wavelength, emission wavelength, emission intensity, average size, size distribution, shape and surface state.
2. Description of the Related Art
Group II-VI, III-V or IV-VI compound semiconductor nanocrystals are peculiar materials whose optical and electrical properties vary depending on the size, shape and surface state of the particles due to a quantum confinement effect. In particular, semiconductor nanocrystals are advantageous because the band gap of a semiconductor may freely change within a wide energy range from UV light to IR light depending on the size and shape of the particles. Thanks to such advantages, semiconductor nanocrystals are receiving attention as a novel absorption/emission element in a variety of fields including biological labeling, LEDs, lasers, solar cells, and wireless communication. As such, the value of technology for reproducibly synthesizing semiconductor nanocrystals having a desired shape and controlling properties thereof is gradually increasing.
Typically semiconductor nanocrystals are manufactured using a colloidal growth method. The colloidal growth method is a process that induces the growth of semiconductor nanocrystals by reacting a metal precursor with a chalcogen precursor in an organic solvent such as tri-n-octylphosphine oxide or trioctylphosphine or a polar solvent such as water. In the course of growing semiconductor nanocrystals, depending on conditions including the type of solvent and ligand, the type and concentration of the precursor, and the temperature and time required to grow nanocrystals, semiconductor nanocrystals having various shapes such as a sphere, rod, bi-, tri-, or tetra-pod may be manufactured to a desired size.
However, when using only the colloidal growth method as described above, it is difficult to control the surface state of semiconductor nanocrystals, and thus the properties of semiconductor nanocrystals are difficult to reproduce and also limitations are imposed on changing the shape of semiconductor nanocrystals. For this reason, thorough research and development into a surface etching method which may be utilized together with the method of growing semiconductor nanocrystals in order to more precisely control the shape and properties of semiconductor nanocrystals is ongoing.
In the case of bulk semiconductors, surface etching methods using various halogen compounds such as hydrogen chloride or trichlorophosphine are disclosed (U.S. Pat. Nos. 5,242,468, 5,869,398, and 5,888,906). However, such surface etching methods are problematic because they use a gas-phase reaction and thus are difficult to apply to most semiconductor nanocrystals synthesized in a solution phase.
In order to etch semiconductor nanocrystals in a solution phase, the following methods have been proposed: 1) van Dijken and Torimoto et al. developed a photochemical etching method wherein various types of group II-VI or IV-VI compound semiconductor nanocrystals are photoexcited in an oxygen-containing aqueous solution thus inducing the oxidation of chalcogen (van Dijken et al., Chem. Mater. 10:3513 (1998), Torimoto et al., J. Phys. Chem. B105:6838 (2001), U.S. Ser. No. 07/006,5665 and EP 1 491 502); 2) Meulenkamp and Liu et al. studied the procedure of etching ZnO and PbS nanocrystals using acids such as acetic acid and hydrochloric acid in a polar solvent such as alcohol or water (Meulenkamp et al., J. Phys. Chem. B102:7764 (1998) and Liu et al., Geochim. Cosmochim. Ac. 72:5984 (2008)); 3) Li et al. introduced the etching of CdSe nanocrystals using oxygen and aminoalcohol in water (Li et al., J. Am. Chem. Soc. 127:2524 (2005) and Li et al., Adv. Funct. Mater. 16:345 (2006)]); 4) Yu and Liu et al. reported the etching of CdSe nanocrystals using a strong acid such as hydrochloric acid or a peroxide such as benzoyl peroxide or hydrogen peroxide in chloroform (Yu et al., Chem. Mater. 15:2854 (2003) and Liu et al., Inorg. Chem. 47:5022 (2008)); and 5) Galian et al. reported the photochemical etching of CdSe core and CdSe/ZnS core/shell nanocrystals using oxygen in toluene, aromatic ketone, and excitation light (Galian et al., J. Am. Chem. Soc. 131:892 (2009)).
However, the above methods of etching semiconductor nanocrystals in a solution phase mainly use an aqueous solution or a polar solvent such as alcohol and thus cannot be directly applied to most semiconductor nanocrystals synthesized in a non-aqueous organic solvent. Furthermore, the average etching procedure is a very slow reaction that takes place over the time span of from ones to tens of hours. Furthermore, the etching methods using strong acid, peroxide, or oxygen-aromatic ketone-excitation light in a non-aqueous solvent have a reproducibility of the reaction rate and thus cannot be utilized as an effective etching process.
Among such methods of etching semiconductor nanocrystals, the photochemical etching method in which the nanocrystals are photoexcited to thus induce the chemical etching reaction may induce size-selective etching because of the properties of semiconductor nanocrystals whose absorption wavelength varies depending on the particle size, and is thus regarded as effective for precisely controlling the properties of semiconductor nanocrystals.
Currently, the most advanced research into photoetching of semiconductor nanocrystals is being conducted by some researchers including Torimoto. They developed a photochemical etching method in which group II-VI or IV-VI compound semiconductor nanocrystals are photoexcited in an oxygen-containing aqueous solution or atmosphere thus inducing the oxidation of chalcogen (Matsumoto et al., J. Phys. Chem. 100:13781 (1996), van Dijken et al., Chem. Mater. 10:3513 (1998), Uematsu et al., Nanotechno. 20:215302 (2009), U.S. Ser. No. 07/006,5665 and EP 1 491 502). In addition, there was reported a photochemical etching method using monochromatic excitation light such as a laser to increase size-selectivity (Torimoto et al., J. Phys. Chem. B105:6838 (2001) and Torimoto et al., J. Phys. Chem. B110:13314 (2006)). In addition, research results have been published in which complicated structures such as CdS nanorods, silica or organic polymer shell clad core/shell nanocrystals, and an array of two-dimensional nanocrystals are photoexcited (Torimoto et al., J. Nanosci. Nanotechno. 9:506 (2009), Iwasaki et al., J. Phys. Chem. B 108:11946 (2004), Yoon et al., Mater. Res. Bull. 41:1657 (2006) and Chen et al., J. Am. Chem. Soc. 131:18204 (2009)).
However, the above photoetching methods are problematic because they are based on oxidation by oxygen in an aqueous solution phase and thus cannot be directly applied to most semiconductor nanocrystals synthesized in a non-aqueous organic solvent, and also because the reaction is very slow, so slow in fact that high-output excitation light of about tens of mW should be applied for ones of hours or longer in order to cause a blue shift of about 10 nm (Torimoto et al., J. Phys. Chem. B 105:6838 (2001)). Also, in the case of CdS nanorods, because only anisotropic etching in which long nanorods are shortened during photoetching is possible, isotropic etching in which the size is reduced while the shape is maintained cannot be performed (Torimoto et al., J. Nanosci. Nanotechno. 9:506 (2009)).
Recently, Galian et al. published a photoetching method not in aqueous solution/oxygen conditions but in toluene/oxygen conditions, but the reaction was difficult to control and the reproducibility was low and thus this method is not regarded as effective (Galian et al., J. Am. Chem. Soc. 131:892 (2009)).
Taking into consideration the above problems, the present inventors have devised a method of effectively etching semiconductor nanocrystals synthesized in a non-aqueous solvent using any halogenated solvent such as dichloromethane, chloroform or tetrachloromethane, containing phosphine such as tributylphosphine or triphenylphosphine, or a primary amine such as propylamine, octylamine or oleylamine.
In this method, 1) halide ions which are a reaction product of phosphine and halogenated solvent function to induce a chemical etching reaction for removing metal ions from the surface of semiconductor nanocrystals, thereby rapidly and reproducibly controlling the shape and properties of semiconductor nanocrystals. Furthermore, the chemical etching reaction according to the present invention enables anisotropic etching which is performed at different rates depending on the surface facet of semiconductor nanocrystals, and thus may be usefully employed to control the shape and properties of semiconductor nanocrystals. In addition, 2) the halogenated solvent receives electrons from the photoexcited semiconductor nanocrystals thus producing halide ions and also induces a photoetching reaction for removing metal ions from the surface of semiconductor nanocrystals along with a primary amine, thereby rapidly and reproducibly controlling the shape and properties of semiconductor nanocrystals. Furthermore, the photoetching reaction according to the present invention enables isotropic etching in which the shape of anisotropic nanocrystals such as semiconductor nanorods is maintained and the size thereof is reduced, and thus may be utilized as a novel method of controlling the shape and properties of semiconductor nanocrystals.